Conventional temperature measuring apparatuses or thermometers are often located in an outdoor location, where they are exposed to ambient air, and, in many cases, sunlight. Conventional thermometers are intended to record or display ambient air temperature. In many situations, however, they actually record or display the temperature of their sensing element, which may or may not correspond to the ambient air temperature. For example, when the sensing element is exposed to sunlight, the sensing element may be 20° F. to 30° F. above the ambient temperature. Even when placed in the shade, the sensing element can receive reflected solar radiation or infrared radiation from sunlight-heated surroundings, which can cause the sensing element to experience a temperature a few degrees higher than the ambient air temperature.
Measured temperatures can be combined with other measurements in order to determine additional environmental conditions. For example, a temperature measurement can be combined with a wind velocity measurement in order to determine a wind chill measurement. Wind chill is an attempt to relate a person's or animal's heat loss at a temperature and a specific wind velocity to the same temperature with no wind velocity. Typically, wind chill measurements are only applied to temperatures below 50° F. and winds above 3 miles per hour. Cooling effects of wind, however, continue at temperatures above 50° F., and therefore the term “wind effect” can be used to express both normal wind chill measurements and cooling effects of wind outside of the normal range of wind chill measurements.
Wind effect is determined based on surrounding temperature and wind velocity. There are many different methods of determining wind velocity. For example, a standard method for determining wind velocity uses an anemometer, in which moving air (wind) causes an object (e.g., an impeller) to rotate. Wind velocity is determined based on the speed of rotation of the object. The rotating object, however, experiences inertia, friction, and wear, which can affect the use or accuracy of the anemometer.
Acoustics can also be used to determine wind velocity by measuring the time required for sound waves to travel between two or more points or receivers. Using acoustics, however, requires either the additional cost of multiple receivers or moving parts in order to orientate transducers in the wind.
Another method of determining wind velocity uses air pressure in which an object that is placed perpendicular to moving air has a higher pressure on the side facing the wind and a lower pressure on the side facing away from the wind. The difference in pressures on the sides of the object is used to determine wind velocity. However, similar to the anemometer, the object placed in moving air introduces a moving part that experiences inertia, friction, and wear. In addition, the object must be orientated with respect to the wind in order to accurately calculate wind velocity.
A further method of determining air or wind velocity uses the cooling effect of wind on a heated wire in which the moving air changes the resistance of the heated wire. Wind velocity is calculated based on the change in resistance of the wire. This method requires a constant, sizable, and stable source of power (e.g., 120 volts AC) to heat the wire and, therefore, is not practical for sustained operation with limited sources of power, such as a battery, solar power, etc.